add workspace_hack dependency to zenith_utils

I didn't think this mattered, but it does: if you add a dependency to
zenith_utils, but forget to request a feature you need, the crate will
build from the workspace root, but not by itself.

It's probably better to pull in the whole dependency tree.

This leaves one problem unsolved: the missing feature above will now be
a latent bug. If that feature gets removed later by other crates, and
then the workspace_hack Cargo.toml is updated, this missing feature will
become a build failure.

Cargo.lock

@@ -2462,4 +2462,5 @@ dependencies = [
  "hex-literal",
  "serde",
  "thiserror",
+ "workspace_hack",
 ]

zenith_utils/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2018"
 serde = { version = "1.0", features = ["derive"] }
 bincode = "1.3"
 thiserror = "1.0"
+workspace_hack = { path = "../workspace_hack" }
 
 [dev-dependencies]
 hex-literal = "0.3"